Influence of the Fluid-to-Film Transition on Photophysical Properties of MLCT Excited States in a Polymerizable Dimethacrylate Fluid

• Published in: The journal of physical chemistry. B 2011-01, Vol.115 (1), p.64-70
• Main Authors: Knight, Troy E, Goldstein, Anna P, Brennaman, M. Kyle, Cardolaccia, Thomas, Pandya, Ashish, DeSimone, Joseph M, Meyer, Thomas J
• Format: Article
• Language: English
• Subjects: B: Statistical Mechanics, Thermodynamics, Medium Effects
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp107077t

Photophysical properties of the salts [Ru(bpy)3](p-Tos)2, [Ru(dmb)3](PF6)2, [Ru(vbpy)3](PF6)2, and [Ru(phen)3](p-Tos)2 (bpy = 2,2′-bipyridine, dmb = 4,4′-dimethyl-2,2′-bipyridine, vbpy = 4-methyl-4′-vinyl-2,2′-bipyridine, phen = 1,10-phenanthroline, and p-Tos = p-toluene sulfonate) in fluid and film polyethylene glycol dimethacrylate containing nine ethylene glycol spacers (PEG-DMA550) are reported. MLCT absorption energies and bandshapes are similar in fluid and film PEG-DMA550 pointing to similar local dielectric environments, presumably dominated by the polar acrylate groups. Emission energies and excited-to-ground state 0−0 energy gaps (E 0), determined by emission spectral fitting, are blue-shifted, and band-widths-at-half height (Δv̅ 0,1/2) decreased, due to an expected “rigid medium effect” in PEG-DMA550 film. The extent of loss of medium dipole reorientation in the rigid environment, and the increased emission energies in the film, resulted in enhanced emission quantum yields and excited state lifetimes in accordance with the energy gap law. The “rigid medium effect” in PEG-DMA550 is less pronounced than in films of poly(methyl methacrylate) (PMMA) pointing to a more fluid-like local environment presumably arising from the ethylene glycol linker spacers in PEG-DMA550. Comparison of the absorption, emission, emission spectral fitting, and emission lifetime results for [Ru(dmb)3](PF6)2 and [Ru(vbpy)3](PF6)2 shows that the vinyl groups of vbpy copolymerize with PEG-DMA550 covalently incorporating Ru(vbpy)3 2+ as a cross-linker into the polymer network. The most dramatic effect of the fluid-to-film transition is seen in the emission lifetime data for [Ru(phen)3](p-Tos)2, with an increase of ∼3 in the PEG-DMA550 film. Ru(phen)3 2+ cations appear to occupy a low symmetry site in the films probably close to the polar acrylate groups in a structurally confined environment.